Buildings, towers and similarly heavy structures commonly are built on and around a steel framework. A primary element of the steel framework is the joint connection of the beams to the column. Gusset plates have been used to provide a superior beams-to-column, moment-resisting joint connection, as set forth in my related U.S. Pat. No. 5,660,017 entitled Steel Moment Resisting Frame Beam-to-Column Connections. A brace, which further strengthened the steel framework, was later added to that joint connection by connecting a brace or braces to the gusset plates, as set forth in my U.S. Pat. No. 6,516,583, entitled Gusset Plate Connections for Structural Braced Systems. Additional, related patents issued to me are U.S. Pat. No. 6,138,427 for Moment Resisting, Beam-to-Column Connection and U.S. Pat. No. 6,591,573 for Gusset Plates Connection of Beam-to-column.
The above patents teach placing a pair of gusset plates opposite each other, on opposite sides of a column, with the gusset plates extending outwardly from the column along the sides of a beam, to provide a means for connecting the beam to the column, except my U.S. Pat. No. 6,591,573, which does not use gusset plates extending across a column and, is, therefore, excluded as one of “my related patents”, hereinafter. Of course, as taught in my U.S. Pat. No. 5,660,017 and my related patents, the gusset plates may extend in both directions from a column, that is, they may extend across a column, and connect two beams to one column, one beam on each side of the column. Such patents also teach gusset plates, welded to the column along the vertical flange edges of the column, and those gusset plates also, are welded to the beam along the horizontal flange edges of the beam, in the longitudinal direction of the beam, or, alternatively, if the beam's flanges are not as wide as the column, and, so, the beam's flanges do not span the width between the gusset plates, sufficiently wide cover plates are attached to the flanges of the beam, and the gusset plates are welded to the longitudinal edges of such cover plates, in the longitudinal direction of the beam. Thus, a longitudinal weld connection lies along the gusset plates in the longitudinal direction of the beam. The gusset plates are thus fixedly attached with respect to the beam.
Fillet welds are preferably used both in attaching the gusset plates to the vertical flange edges of the column and in the longitudinal welds attaching the gusset plates to the beam or, alternatively, to cover plates attached to the beam.
The teachings of those patents are incorporated herein by this reference, particularly U.S. Pat. No. 5,660,017 which teaches the original concept of using gusset plates to provide great overall strength and ductility in beams-to-column joint connections and in beam-to-beam connections across a column. Such a gusset plates connection from beam-to-beam remains effective across a damaged column, even if the column provides no support, and also, remains effective across a compromised beam-to-column-to-beam joint connection. As explained hereafter, the gusset plates beam-to-beam connection, of the invention herein, also remains effective under such circumstances.
The gusset plate inventions described in the above-mentioned patents were occasioned by the poor performance of the “traditional”, prior fabrication of beam-to-column joint connections, wherein, customarily, the beam was connected to a column by welding the ends of the beam flanges to the column flange, (column face), using full penetration, single bevel groove welds to obtain a moment-resisting connection. When such prior connections were loaded by severe moments and loads such as those caused by earthquakes, explosions and other disasters, they failed. The Northridge earthquake in California in 1994 demonstrated that such prior joint connections were unsuitable for resisting or carrying, (transferring), moments and loads caused by earthquakes. Therefore, such “traditional joint connections” were also unsuitable in the event of explosions, tornadoes and other disastrous events. Under severe load and moment conditions, occasioned by such a disastrous event, the forces and loads of the event would cause the “traditional joint connection” to fail. There occurred one or more of, fracture of the welds, fracture of the metal of the beam or of the column, or the beam pulled divots out of the flange, (face), of the column.
There was insufficient strength, insufficient resistance to moments and insufficient ductility in the prior joint connections. Prior construction had little or no continued strength beyond the yield point of the joint connections.
Over the last several years, there has been considerable additional concern as to how to improve the beams-to-column joint connections so they will withstand explosions, blasts and the like as well as other related load phenomena. Of particular concern is the prevention of progressive collapse of the building if there are one or more column failures due to terrorist bomb blast, vehicular and/or debris impact, structural fire attack or any other impact and/or heat-induced damaging condition.
Column failures due to explosions, severe impact and/or sustained fire, have led to progressive collapse of entire buildings. An example of such progressive collapse occurred in the bombing of the A. P. Murrah Federal Building in Oklahoma City in 1995 and the aerial attack of the World Trade Center towers in 2001.
It is to be appreciated that U.S. Pat. No. 5,660,017 teaches that gusset plates may be used to attach beams, on both sides of a column, to the column. In other words, a single pair of gusset plates may extend across the column and along each beam on opposite sides of the column. Not only are the beams strongly connected to the column by the gusset plates, but, the beams are also strongly connected to each other by the gusset plates.
Following the 1994, California earthquake, in addition to my invention set forth in U.S. Pat. No. 5,660,017, a number of other alternatives, to resist joint connection failure, were adopted for use in steel construction design for improved seismic performance; for example, the reduced beam section, (RBS), or “dogbone” joint connection, in which the beam flanges are narrowed near the joint connection. This alternative design reduces the plastic moment capacity of the beam allowing inelastic hinge formation of the beam to occur at the reduced section of the beam, in order to relieve some of the stress in the joint connection between the beam and the column. U.S. Pat. No. 5,595,040, issued Jan. 21, 1997, for Beam-to-Column Connection to Sheng-Jin Chen, illustrates such “dogbone” connections. It works. Nevertheless, inasmuch as the plastic moment capacity of the beam is reduced, because of the narrowing of the beam flanges, the moment load which can be withstood by the beam is substantially reduced.
Another alternative is illustrated by U.S. Pat. No. 6,237,303, issued May 29, 2001, to Clayton Jay Allen et al., in which slots and holes are used in the web of one or both of the column and the beam, in the vicinity of the joint connection, to provide improved stress and strain distribution in the vicinity of the joint connection.
Other post-Northridge joint connections are also identified in FEMA 350-Recommended Seismic Design Criteria for New Steel Moment Frame Building, published by the Federal Emergency Management Agency in 2000. All such post-Northridge joint connections have reportedly demonstrated their ability to achieve the required inelastic rotational capacity to survive a severe earthquake or other disastrous event.
None of these alternative joint connections, however, provide independent beam-to-beam structural continuity across the column; such continuity being capable of independently carrying gravity loads under a “double-span” condition resulting from a column violently removed by, say, explosion, blast, impact or other means, regardless of the damaged condition of the column. Indeed, there are no additional load paths across the column in the event of column failure or joint connection failure or both. Nor do any of these alternatives, except my gusset plates used as taught in U.S. Pat. No. 5,660,017 and my related patents listed above, and the gusset plates used as taught in the invention herein provide any significant torsional capacity or significant resistance to lateral bending to resist direct air blast impingement and severe impact loads. Torsional demands are created because the top flange of the beams is typically rigidly attached to the floor system of a building laterally, thereby leaving the bottom flange of the beam free to twist when subjected to, say, direct lateral air blast impingement caused by a terrorist attack.
Nor do the alternative joint connections provide any reserve capacity for resisting inelastic axial tension load demands imposed by the beams in a “double-span” condition following the removal or impairment of a column or loss or impairment of beam-to-column joint connections, notwithstanding the alternative joint connections rated inelastic rotational capacity.
These collective attributes do not exist in prior art beam-to-column joint connections.
All of the aforementioned missing attributes, if included, would clearly mitigate the likelihood of progressive collapse of steel frame buildings and would provide blast hardening of beams-to-column joint connections against terrorist attack.
All of these post-Northridge alternative joint connections and pre-Northridge joint connections, (except those of my U.S. Pat. No. 5,660,017 and my related patents), may be classified as “traditional joint connections” because they rely on direct welding of the beam flanges or a beam's cover plates, to the face of the column flange. Thus, the “traditional joint connections” cannot maintain beam-to-beam continuity across a blast-damaged, or otherwise failed, column, because such continuity necessarily depends on maintaining the structural integrity of that very same column and the joint connections thereto. Therefore, such beam-to-beam continuity is lost when the column has been either altogether removed or, as a minimum, the column or the joint connections thereto, have been severely damaged and structurally compromised.
Nor can such “traditional joint connections” maintain their rated inelastic rotational capacity upon a blast and its resulting effects or similar damaging effects, because the “traditional joint connections” provide no protection of the joint connection as provided herein by the robust capacity of the gusset plates.
Simply put, the gusset plates of the present invention also provide a shield for the joint connection against blasts and its effects. Such feature is also found in my U.S. Pat. No. 5,660,017 and my related patents.
The “traditional joint connections” are fundamentally not able to satisfy the performance expectations for credible mitigation of blast effects. Also, in such connections, an essential, suitable, beam-to-beam structural linkage across a blast-failed column and/or its beam-to-column joint connections, if impaired or lost, simply does not exist.